1. Field of the Invention
The present invention relates generally to the art of accoustical defect detection and specifically to an ultrasonic inspection apparatus and method for locating defects in eccentric wall tubular goods, such as oil and gas country pipe.
2. Description of the Prior Art
Ultrasonic transducers are known which have been used in pulse-echo mode to locate flaws and defects in tubular goods. In the pulse-echo mode, the ultrasonic transducer emits an ultrasonic wave and then waits to receive an echo from a defect. The angle of incidence and angle of reflection relative to the surface of the defect must be approximately equal. As a result, a transmitting transducer can only receive an echo from a defect surface which is approximately normal to the direction of ultrasonic wave transmission. If the defect surface is more than about five degrees off-normal to the direction of propagation, the ultrasonic wave will be reflected but will not return a sufficiently large component to the transmitting transducer for the defect to be detected.
Ultrasonic transducers have also been used in the past in pulse-echo mode to generate ultrasonic shear waves which travel peripherally around the tubular goods being examined, and to detect echoes reflected peripherally back to the transducer. Axially oriented ultrasonic transducers have also been used to generate axial shear waves and to detect axial echoes. For some purposes, ultrasonic transducers have been oriented perpendicular to the examined surface, for instance to determine wall thickness, and have been operated in a pulse-echo mode.
A three dimensional defect commonly has at least some surface portion which is normal to one of the pulse-echo operated transducers, so that the defect can be detected. However, a two dimensional defect, such as a crack, can only be detected by pulse-echo transducers which are oriented substantially perpendicular to the surface of the crack. As a result, peripherally oriented pulse-echo transducers and axially oriented pulse-echo transducers are only able to detect cracks which are substantially parallel or perpendicular to the axes.
Certain of the prior art devices have attempted to provide a wider range of coverage by orienting selected transducers at various angles, e.g. forty-five degrees. However, because the direction of propagation must be within five degrees of normal to a crack to be assured of detection, a wide range of wave propagation directions would be required for assuring that cracks would not go undetected.
Inspection of oil and gas country tubular goods causes particular problems when using ultrasonic devices because the pipe is never perfectly concentric. The American Petroleum Institute (API) has written specifications for the limits on which oil and gas country tubular goods can be eccentric. The specifications define a "nominal" wall based upon a weight per foot for a given size diameter pipe. Thus, for a 7 inch outside diameter pipe of 49.50 pounds per foot, the nominal wall is calculated to be 0.730 inches thick. For a 7 inch outside diameter pipe of 66.50 pounds per foot weight, the nominal wall is calculated to be 0.980 inches. The API specifications basically allow the nominal wall to be reduced by only 121/2% of nominal before being rejected. The same specifications provide that the outside-diameter may not exceed 3/4 of 1% of the specified outside diameter. In addition, the inside diameter of the pipe must drift, or pass a drift mandrel, of a specific size. For 7 inch pipe of 49.50 pounds per foot weight, this drift diameter is 5.415 inches. From these figures, an allowable wall thickness can be calculated. For instance, for 7 inch pipe of 49.50 pounds per foot weight, the allowable wall can range from about 0.638 inches to 0.818 inches.
In order to overcome shortcomings of the pulse-echo technique, certain prior devices used "thru-transmission" sometimes referred to as pitch-catch techniques. In this technique, a sender element is triggered to transmit an ultrasonic shear wave which is "caught" by a receiver element. The method was generally limited to materials having sufficiently small changes in wall tolerance to allow the receiver transducer to be large enough to catch the incoming ultrasonic signal. Since the prior art devices usually used "point focus" transducers, this method was limited to thin wall tubular goods.
The present invention has as its object a device and method for overcoming the above referenced problems and for providing an ultrasonic inspection system which detects defects and cracks oriented at a wide variety of orientations in the object being examined.
Another object of the invention is to provide an apparatus with a receiving element having a receiving surface which is linear over a larger area than prior devices to receive the incoming signal from the sending element from eccentric wall pipe which varies by the limits specified by API and yet be able to locate defects without false alarms caused by large changes in wall thickness.